Care UK has paid £48m for England's biggest out-of-hours GP service, Harmoni, in a deal to create a new private health concern that could treat 15 million patients.

Harmoni, a former GP co-operative, was the largest private firm providing out-of-hours services and won 12 NHS contracts to run the new non-emergency 111 health phone lines – beating a joint bid from Care UK and Capita. Last year it had revenues of £100m from the NHS.

Care UK recently lured a top civil servant from the Department of Health – Jim Easton, who oversaw the NHS 111 procurement process – to become its managing director.

Care UK said the deal was about making "sure that patients get the right treatment at the right place without unnecessarily occupying hospital beds".

Mike Parish, the chief executive of Care UK, said: "We understand the urgency with which the NHS needs to be able to reduce unscheduled and inappropriate hospital admissions of patients who can and should receive treatment elsewhere."

Harmoni began as a GP co-operative in north-west London in 1996. Care UK now runs more than 50 primary care services – including GP and walk-in services, out-of-hours and diagnostics centres – and six hospitals that carry out NHS work.